Elastomer-covered roller having a thermally sprayed permeable bonding material

ABSTRACT

A elastomer-covered roller ( 10 ) is provided with an improved bond coating for bonding the elastomeric cover ( 14   b ) to the core ( 11   b ). At least one layer of material ( 21 ) is thermally sprayed on the core ( 11   b ) to form a rough surface having a roughness from 400 microinches R a  to 2000 microinches R a  or greater. At least one layer ( 21, 25, 26, 31, 32 ) is either sufficiently permeable to divert bond-degrading fluids away from the bond line ( 22, 27, 33 ) or is impermeable to bond-degrading fluids.

CROSS-REFERENCE TO RELATED APPLICATION

This is a divisional of U.S. Pat. appl. Ser. No. 09/740,470, filed Dec.19, 2000, now U.S. Pat. No. 6,523,262, which is a continuation-in-partof U.S. patent appl. Ser. No. 09/570,250 filed May 12, 2000, now U.S.Pat No. 6,394,944.

TECHNICAL FIELD

The invention relates to the manufacture of elastomer-covered rollers.

DESCRIPTION OF THE BACKGROUND ART

Rollers with covers of elastomeric material are used in a wide varietyof industrial applications. One process is a plastic film manufacturingprocess known as cast film extrusion. Films of thermoplastic polymerssuch as polyvinylchloride and polycarbonate are made by this process.The most popular polymers made by this process are polyethylene andpolypropylene.

In a typical application, molten polymer from an extruder is droppedinto a nip formed between a chrome-plated steel roller and anelastomer-covered steel roller. One common type of elastomer used inthis application is silicone rubber, although Hypalon™, available fromDu Pont, and other polymer materials can also be used. The chrome-platedsteel roller and the elastomer-covered roller are normally water cooled,or otherwise chilled, since the molten plastic may be at a temperatureof several hundred degrees Fahrenheit. The chrome-plated roller quicklychills the plastic below its melting point and the nip formed with theelastomer-covered roller determines the gauge (thickness) and thesurface finish of the film. The flexibility of the rubber coveringcompensates for minor machine misalignment and other variables, andprovides a wider nip than two hard surfaced rollers.

In some applications, the film manufacturer also runs the surface of thesilicone roller through a water bath to provide a measure of release(non-stick) from the tacky semi-molten plastic, and to provideadditional cooling of the rubber surface. The excess water is removedwith a doctor blade or other device and most of the remainder quicklyevaporates from the hot roller surface. A small amount of the water (orwater vapor) does pass through the extrusion nip. Because of thetemperature difference between the rubber roller surface and thecore/rubber bond line, water vapor tends to migrate through the rubbercovering and condense at the core surface. Although the interfacebetween the core and other materials is an area, this will berepresented by the term “bond line” which is the portion seen in thedrawings.

The vapor pressure of water at the hot roller surface may be several psi(pounds per square inch), while the vapor pressure at the core surface(typically 40 to 70° F.) is a fraction of one psi. This pressuredifference drives the water vapor through the wall of the siliconerubber covering. Silicone rubber, in particular, is quite permeable towater vapor. Other polymers that may be used for the rubber covering,such as Hypalon, are either less permeable to water vapor or are morehydrophilic and absorb the water. These polymers are still susceptibleto water vapor migration. The rate of migration is slower, however.

As used herein, the terms “permeable” and “impermeable” shall meanpermeable and impermeable to moisture or to DOP-type plasticizers.“Moisture” shall mean water or water vapor, or both. Moisture andDOP-type plasticizers shall be included in the term “bond-degradingfluids”, and the term “fluids” shall include liquids and vapors.

Even in applications where the rubber nip roller is not run through awater bath, migration of water vapor to the core is still known to be aproblem, especially if the core temperature is below room temperature orthe dew point. Rollers used for the application of coatings frequentlyhave problems with water accumulation at the rubber/core bond line. As aresult, rollers applying water-based coatings are known to havecorrosion and bond failure at the core. The water vapor migrationproblem is not limited to cast extrusion nip rollers.

Silicone rubber is a material that may be used as the cover in theelastomer-covered roller described for the above application. Siliconerubber is, however, a material that is difficult to bond to a metalroller core on a consistent basis. It is also difficult to maintain thebond in applications where the roller is used at high temperature andhigh pressure. In some cases, the silicone rubber may peel cleanly offthe metal core without leaving any residue of rubber, indicating a lossof bonding. Loss of bonding in one area requires that theelastomer-covered roller be replaced.

Under high pressure, typically 100 pli (pounds per linear inch) or more,there is a stress concentration at the rubber-to-metal bond line causingthe silicone to tear away from the core due to shearing forces. The bestcurrently available method to achieve a consistent bond is to grit blastthe surface of the core to a high roughness, about 500 R_(a), beforeapplying the bonding agents or primers to the core. Bonding agents areapplied in thin layers, typically less than 1 mil.

Grit blasting has certain limitations, however. It is difficult toachieve surface roughnesses greater than 500 R_(a). Even maintaining 500R_(a) generally requires the constant use of new grit and attention tothe grit blasting process variables. The grit-blasted core surface has ahigh R_(a) but it is not a complex, high profile, surface like Velcrowhich not only has a high surface area, but also has a lot of “hooks”.So called “hooks” improve the bond strength by trapping material withinsmall surface features.

If a rougher surface were available, for example, in the 500 to 2000R_(a) range, not only would the surface area available for bonding begreatly increased, but the location of the rubber-to-metal bond linewould be thicker and more diffused. This would in turn diffuse theshearing forces trying to tear the rubber from the core. Simply toolinga thread pattern into the core does not achieve this goal, because thedirection of the thread is nearly parallel to the applied forces in theroller nip rather than perpendicular to them.

An improved bonding surface is needed for elastomers that are weakly orinconsistently bonded, especially if the covered rollers are exposed tohigh temperature or pressures, or high moisture conditions. The bondingsurface must provide a high surface area and surface roughness and yetbe easily and consistently produced. A very high surface roughness willalso diffuse the stress at the bond line improving the longevity of therubber to metal bond.

Polyurethane is another material that can be used for the elastomericcover in the present invention. For bonding polyurethane to supportinglayers, primary reliance has been placed on chemical bonding, to beassisted by mechanical bonding. As with silicone-based materials, thelimit of surface roughness available with current methods of mechanicalbonding is about 500 microinches R_(a).

In addition, water vapor easily permeates through either a siliconerubber layer or a polyurethane layer, and corrosion may occur at thebond line due to the collection of water vapor there. Moisture willmigrate into the outer roller cover if the roller is either chilled orexposed to water in the application. It would be beneficial to provide abond coat or layer that is resistant to such corrosion, as well as oneproviding a stronger mechanical bond.

Another application of rollers is in the embossing and calendering ofpolyvinylchloride (PVC) film. The PVC film is made from a rigid PVCresin and is plasticized with oils to produce a softer material. PVCfilm is typically plasticized with DOP (dioctyl phthalate, a syntheticester type oil) or chemically similar materials. These forming typeoperations require that the film be heated in a range of from 350° F. to400° F. The film can lose DOP at these temperatures due to migration andevaporation.

In either embossing or calendering, the arrangement of rollers issimilar to cast film extrusion with a hard surface roller forming a nipwith a rubber-covered roller. Due to high temperatures, therubber-covered roller is typically internally water cooled. Thetemperature differential through the rubber cover creates the same typeof migration problem with DOP as is seen with water vapor in the castfilm process. DOP migrates through the rubber covering to the core whereit can degrade the bond between the core and the rubber cover. Thehydraulic force of the oil tends to break the bond between the core andthe cover layer of material. This is a second type of bond degradation,in addition to corrosion by moisture described above.

This can occur in rollers with silicone rubber covers, as well asrollers with a multi-layer configuration using EDPM rubber at the core.These materials may be permeated by DOP-type plasticizers.

Therefore, a general object of the invention is to improve bondingtechniques and protect the bond layer against these adverse effects ofits operating environment.

SUMMARY OF THE INVENTION

The invention concerns a method of making a roller that includesthermally spraying a bond coat to substantially cover a portion of thecore on which an elastomeric layer is to be bonded and in which the bondcoat provides a surface roughness to assure a strong mechanical bond.This bond coat has at least one coat that protects the bond line, eitherby allowing fluids to migrate away from the bond line, and/or byshielding the core from corrosion caused by the condensation of fluidsnear the surface of the core.

The invention also relates to a roller resulting from the utilizing thismanufacturing method.

The bond coat comprises a thermally sprayed material selected from agroup of materials consisting of metals, metal alloys, ceramics andcermets.

In the prior art, core surfaces were prepared by sanding or blasting themetal core prior to the application of chemical bonding agents. Thesemethods do not always produce the desired level of mechanical bonding.Thermal spraying has been known in the manufacture of ceramic coveredrollers, but has not heretofore been applied to bond elastomeric layersto a metal core.

The invention provides improved constructions of elastomer-coveredrollers. These elastomers include silicone, EPDM(ethylene-propylene-diene-monomer), urethane elastomers and othersynthetic or natural rubber elastomers. Although the invention isdescribed in examples in which the elastomeric layer is the outer layer,it would also be possible to add layers outside the elastomeric layer.

The invention may be practiced in further aspects by providing atwo-layer bond coat in which an outer layer is permeable to allowmigration of water and water vapor, while the layer next to the core isimpermeable to provide a corrosion barrier.

In another embodiment two layers of permeable material are applied tothe core, with the inner layer being provided for better bonding to thecore and the outer layer being provided for better bonding to theelastomeric layer.

Other objects and advantages of the invention, besides those discussedabove, will be apparent to those of ordinary skill in the art from thedescription of the preferred embodiments which follow. In thedescription, reference is made to the accompanying drawings, which forma part hereof, and which illustrate examples of the invention. Suchexamples, however, are not exhaustive of the various embodiments of theinvention, and therefore, reference is made to the claims which followthe description for determining the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a roller incorporating the presentinvention;

FIG. 2 is a detail sectional view of an embodiment of the prior arttaken in a plane indicated by line 2—2 in FIG. 1;

FIG. 3 is a second sectional view of the embodiment of FIG. 2;

FIG. 4 is detail sectional view of a first embodiment of the inventiontaken in the same plane as FIG. 2;

FIG. 5 is a detail sectional view of a second embodiment of theinvention taken in the same plane as FIG. 2; and

FIG. 6 is a detail sectional view of a third embodiment of the inventiontaken in the same plane as FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, an elastomer covered roller 10 of the presentinvention includes a core 11, with journal shafts 12, 13 extending fromopposite ends of the core 11, and a layer 14 of elastomeric material, inthis example, silicone rubber, which in this example forms the outercover of the roller 10. The core 11 may be made of metal, such as steel,or it may be made of a fiber-reinforced resinous composite material asdisclosed in Carlson, U.S. Pat. No. 5,256,459, issued Oct. 26, 1993.

The elastomeric material may be applied using conventional and knownmethods. Silicone rubber can be applied using methods such as liquidcasting, various forms of extrusion, or by wrapping the core withcalendered sheets of rubber. Urethanes may be applied by liquid castinginto a mold or by a liquid ribbon process described in U.S. Pat. No.5,206,992 and U.S. Pat. No. 5,415,612 assigned to the assignee of thepresent invention.

The problem in the prior art is illustrated in FIGS. 2 and 3. There, alayer of silicone rubber 14 a is bonded to the steel core 11 a at atextured core surface 15 a to which a bonding agent may be applied. Thetexture may be applied to the core surface 15 a by grit blasting or byforming grooves in the surface 15 a of the roller core. When subjectedto temperature differential and moisture as described above, and afteruse, the outer layer 14 a exhibits an uneven roller surface as seen inFIG. 3, and water 16 collects near the core 11 a, at the bond line 15 aseen in FIG. 3, where it causes corrosion or failure of the bonding ofthe outer layer 14 a to the core 11 a.

To correct this problem, a first embodiment of the invention, asillustrated in FIG. 4, includes a permeable bonding layer 21 ofthermally sprayed material, which is applied to a steel core 11 b as abonding surface for the cover material 14 b. This layer 21 is typicallycomposed of a ceramic, cermet, metal, or alloy. The preferred materialsare relatively high in porosity, while retaining structural integrity.These materials also provide rough, complex surfaces after spraying andare easily bonded to rubber at bond line 22. Unlike the simple surfaceprofiles produced by sanding and blasting, thermally sprayed surfacesare more complex and have many surface features that can trap the rubberto form an interlocking bond. Layers produced by the wire arc processare somewhat more complex than sanded or blasted surfaces. Layersproduced by the application of thermally sprayed powders are even morecomplex. Bonding surfaces produced by thermally sprayed powders are usedfor bonding ceramics to metals, a process which is entirely mechanical.When it is desired to bond an elastomer to a metal or alloy core, thebond is produced by a combination of chemical bonding and mechanicalbonding. In that case, it is typical to apply bonding agents and primersover the thermally sprayed bond coat. This allows the elastomer tobecome chemically bonded to the thermally sprayed core.

The surface roughness for robust mechanical bonding of silicone rubberor similar materials is at least 400 microinches R_(a) or more, althougha surface roughness of only 200 microinches R_(a) will provide a bondadequate for many applications (R_(a) denotes average roughness underASME standards and numerical measure in units of microinches.) With thethermally sprayed material, surfaces up to 2000 microinches R_(a) havebeen successfully tested and provide a superior mechanical bond.Ceramics typically provide a surface roughness of less than 300microinches R_(a), if used alone, and do not provide bonding surfaceswhich are as robust as materials providing rougher surfaces. Alumina,alumina/titania, and zirconia ceramics can be used in combination withmetals to increase the texture of bonding surface 22.

The preferred material for the embodiment in FIG. 4 is a metal or alloy,in wire or rod form, applied by a thermal spraying step. The resultingroughness of the bond line 22 is in a range from approximately 500microinches R_(a) to approximately 2000 microinches R_(a). The maximumsurface roughness of a thermally sprayed coating that can currently beproduced is in the 2000 R_(a) range. Please note, that for purposes ofthe explanation, the dimensions of the surface 22, as well as thethicknesses of the layers, have been exaggerated in FIGS. 4, 5 and 6 andare not represented to scale. The roughness of bond line surface 22greatly improves the mechanical bond strength of the rubber-to-metalinterface. One example is Sulzer Metco #2 wire, which is a Nr. 420-type(400 series) stainless steel alloy. 300 Series alloys can also be used.This material can be sprayed at high rates (>25 pounds per hour), is lowin cost, and can be applied in a series of thinner coating layers or ina very thick layer. This material also bonds well to most elastomers. Itprovides the steel roller core 11 b with a measure of protection againstdegrading fluids even though it is somewhat porous. Sulzer Metco #2 wirefor example, applied by the wire arc thermal spray process, for example,can be applied in practical layers up to at least 250 mils in thickness.Thicker layers can be applied but may not be cost effective compared toother means to increase the core diameter.

One of the problems with conventional rubber roller re-covering is thatpart of the core surface is frequently machined off to provide a cleanbonding surface. Over a period of time, the core diameter becomes toosmall to use or the rubber cover thickness increases excessively. With alayer of thermal spray material on the core, it is no longer necessaryto remove part of the core surface to prepare the core surface forrecovering. Part or all of the thermal spray layer can easily bemachined off and replaced. Also, a few mils of the thermal spray layercan be ground off to clean the surface, followed by a thin applicationof the same thermal spray material. A new layer of thermal spraymaterial is easily bonded to an old one. The materials bond welltogether, as long as the sprayed surface is clean and free from greaseand oil. The addition of a thermally sprayed layer can be used to helpcontrol the outer diameter of the core.

Other thermally sprayed materials and methods have also proven toprovide a superior bonding surface according to the present invention.Eutectic 18923 and Sulzer Metco 43C powders, which are both 80/20 alloysof nickel and chromium metals, can be used. These materials, as well asany corrosion resistant thermal spray material, 300 series and 400series stainless steels, nickel chrome alloys, and other nickel-basedalloys, provide both corrosion protection of the core and a suitablebonding surface for elastomers. The surface profiles of most thermallysprayed metals and alloys are very similar for a given particle size inthe starting material. The roughness of the surface formed by a layer ofEutectic 18923 is typically 400 microinches R_(a) while the Sulzer Metco43C provides a surface roughness of about 700 microinches R_(a), ifsprayed with an oxygen-fuel gun device such as the Eutectic Teradyne3000 or Sulzer Metco 6P. The particle size of the 43C is larger than the18923 which accounts for the R_(a) difference. A thickness of thecoating or layer 21 of these materials in a range from 3 to 20 mils hasprovided strong and uniform bonds to an outer layer 14 b of siliconerubber although thinner layers can be used as long as the thickness ofthe coating can be made uniform.

Although the Eutectic 18923 and 43C coatings do not provide as rough abonding surface as the Sulzer Metco #2 wire arc coatings, thesematerials still provide a bond interface having properties superior to agrit-blasted surface having a similar R_(a). Compared to grit blasting,the thermally sprayed surface is more easily and consistently formed aswell.

Under magnification, the #2 wire arc surface appears to be a series ofmountains and valleys. It is more complex than a sanded or blastedsurface. The thermally sprayed powder surfaces produce a more complexsurface that looks like a sintered metal under a microscope. Theelastomer can actually diffuse into the thermal spray coating to thedepth of a few mils.

The roughest surfaces, approximately 1000 microinches R_(a) or greater,provide the best diffusion of stress concentrations at the bondinginterface or boundary 22. The bond interface 22 becomes thicker and morediffused.

If the metal or alloy of the somewhat porous thermal spray layer 21 isdissimilar from the core material, it may be possible to have galvaniccorrosion at the interface between the roller core 11 b and thepermeable layer 21. The two metals and the water (which will containsome electrolytes) will form a battery (in localized areas) andcorrosion will occur at the anode, usually the (steel) core. Even thoughthe core and thermally sprayed layer should be in solid electricalcontact (i.e. a short circuit) and should not form a battery, the oxidespresent in the thermal spray coating can act as an insulator at the coreinterface depending on the material sprayed, the spray equipment, andthe spray parameters.

In order to avoid the possibility of corrosion, the thermal spray layercan be made in two layers of differing porosity levels as illustrated inFIG. 5. The first body of material 25 and the second body of material26, should comprise the same metal or alloy material in order to avoidgalvanic corrosion between the sprayed layers. The first body ofmaterial 25 can be applied by plasma spraying or HVOF (high velocityoxy-fuel) spraying or low velocity flame spraying to produce arelatively dense, pore-free, impermeable coating. It should beunderstood that the coating 25 can be the result of several passes ofthe spray gun which provides several thinner coatings that are mergedinto a thicker coating. The second body of material 26 can be thenapplied by the wire arc step to provide a coarse, porous coating, usingthe techniques described above. A suitable thickness for the first bodyof material 25 is in the range of 3 to 60 mils, but is more preferablyin the range of 10 to 20 mils.

The second body of material 26 can then be applied in a second layerwithout any additional surface preparation. Excellent bonding betweenlayers would be achieved due to the resulting roughness of bond linesurface 27 of the thermally sprayed material 26.

In the case of thermally sprayed powder coatings used as the onlybonding layer 26, a layer of Eutectic 18923 (or other nickel basedcorrosion resistant alloys or stainless steels), 3 mils or more inthickness, generally provides adequate corrosion protection of the corein lieu of the layer 25 of dense metal or alloy material.

A third embodiment of the invention is illustrated in FIG. 6. There,both a wire arc layer 32 and a thermally sprayed powder 31 are rough,porous and permeable but have different structures. Wire arc coatingsare very rough and provide excellent bonding to rubber and diffusion ofstresses at the bond line 33. Thermally sprayed powder coatings are notas rough but have a more regular and predictable porosity network. Inorder to combine the best features of these coatings, a relatively thicklayer (20 to 125 mils) of thermally sprayed metallic powder 31, such asEutectic 18923 or Sulzer Metco 43C, is sprayed on the core 11 d toprovide a permeable layer for diverting fluids away from bond line 33 tothe elastomer and away from the surface of the core. For furtherprotection, however, a relatively thin layer 32 of wire arc material,such as Sulzer Metco #2 wire in the 5 to 15 mil range, is applied overthe metallic powder layer to provide a superior rubber-to-metal bonding.

EXAMPLES

In order to simulate the field conditions that cause rollers to fail bycorrosion and water accumulation, a test device was constructed. Thestandard test roller consisted of a hollow steel tube of about threeinches in diameter with a 80 mil wall thickness. Each bond system,rubber compound, or thermal spray layer to be tested was applied to oneof these steel tubes. The normal tube length was 10 inches.

After the test roller was covered with (silicone) rubber, cured, andground to size, it was placed in a steam autoclave at 180° F. The testroller was mounted on a water cooled mandrel which was maintained atapproximately 50 to 60° F. with a water flow rate of about one gallonper minute. To accelerate the water condensation at the rubber-to-metalinterface, the rubber was only 0.125 inch in wall thickness.

After the test roller was removed from the vulcanizer, the bond strengthof the silicone covering to the core was tested, the covering wasinspected for the presence of water at the bond interface, and the corewas inspected for corrosion.

The quality of the rubber-to-metal bond was determined by the amount ofrubber still attached to the core after the cover was stripped off. Ifthe rubber strip peeled clean from the core, the bond has completelyfailed. If there was a layer of rubber (amount can vary with the bondquality) still attached to the core, the layers of rubber have peeledapart but the bond has not failed.

The standard steel core preparation method provides for either sandingor grit blasting the core surface to about 200 R_(a) (minimum) followedby application of a suitable silicone to metal bonding agent. Withnormal core preparation, water accumulated at the rubber to metal bondinterface and caused blistering and corrosion within 3 to 5 days. Theblisters contained water and were up to 0.5 inch in diameter or larger.The core was corroded and a black sludge, reaction product was alsoformed on the core surface, the same as observed on production rollersreturned from the field. The rubber/metal bond was essentially destroyedand the rubber could be easily peeled like a banana skin. As the rubberwas peeled from the core a significant amount of water appeared at thepeel point showing that the material was saturated.

Increasing the roughness of the core by grit blasting (up to 500 R_(a))improved the bond strength slightly, after the condensation test, butdid not retard the accumulation of water or the formation of corrosion.Part of the bond failure was due to corrosion of the core.

Thermal spray bond layers, 3 to 20 mils thick, of Sulzer Metco 43C,nickel chrome powder (applied with an oxygen/fuel thermal spray gun),were evaluated as bonding layers. After the condensation test, none ofthe test rollers was corroded. Water did accumulate at the bondinterface and form blisters. The remaining bond strength wassignificantly better than any of the sanded or grit blasted rollersbecause of the more complex surface and the lack of corrosion. Theselayers were not permeable enough to prevent the formation of blisters orthe loss of bond strength due to the accumulation of water. The rubberdid not strip cleanly away from the core, however. The amount of rubberleft on the core was related to the surface roughness of the thermalspray coating. Layers of the 43C material had a surface roughness ofabout 700 R_(a) as sprayed.

Thermal spray layers of Eutectic 18923, 3 to 20 mils thick, wereevaluated with similar results to the 43C material. The 18923 materiallayers had a surface roughness of about 400 R_(a) as sprayed.

Thermal spray layers of Sulzer Metco #2 wire, applied with a SulzerMetco 6R wire arc gun, were evaluated in the range of 4 to 60 milsthick. The “medium air cap” was used to provide coatings with surfaceroughness values in the 500 to 2000 R_(a) range. Rougher and more porouscoatings can be produced if the “high profile air cap” is used. Coatingsup to 250 mils thick or more can be produced with the #2 wire arcmaterial. Layers less than about 10 mils thick did not produce fullcoverage of the core surface and were found to be unsuitable for bothcorrosion protection and rubber bonding.

Test rollers with 15 to 60 mil thick layers of #2 arc wire did not haveany core corrosion after the condensation test. On layers that were 30mils thick or less, blisters formed at the bond interface, although theblisters did not form as quickly because the mechanical bond strengthwas greatly improved by the high R_(a) of the bonding surface.

On layers that were 40 mils or thicker, blisters did not form at thebond interface even if the condensation test was extended to nearly twoweeks. As the rubber was pulled away from the core there were dropletsof water at the bond line (the rubber was saturated) but therubber-to-metal bond was still excellent. Both of these observationsshow that condensation was proceeding all the way to the core but thatthe water was being dissipated. Hydraulic forces were not causing therubber to separate from the core, or from itself, and blisters were notforming.

It was therefore determined that a continuous layer of thermal spray,corrosion resistant alloy, even if applied by a low velocity process,will prevent corrosion by water condensation at the rubber to metalinterface.

It was further concluded that a layer of thermal spray metal or alloy asthin as 3 mils will provide a bonding surface equal or superior to ahigh profile (500 R_(a)) grit blasted surface.

It was further concluded that a thermal spray layer of corrosionresistant alloy, applied by the wire arc spray process, is permeableenough to prevent water accumulation due to condensation, if the layeris greater than about 35 mils thick and is sprayed with at least themedium porosity spray parameters (medium air cap), producing coatings inthe 500 to 2000 R_(a) range. Rougher, more porous coatings, producedwith high profile parameters, up to 250 mils thick or more, would alsohave a suitable permeation characteristic.

The above has been a description of the detailed, preferred embodimentsof the apparatus of the present invention. Various modifications to thedetails which are described above, which will be apparent to those ofordinary skill in the art, are included within the scope of theinvention, as will become apparent from the following claims.

We claim:
 1. A roller comprising: a roller core; a bond coat bonded tothe core to provide an outer surface roughness; a coat of elastomericmaterial bonded to the bond coat to form a bond line along which theelastomeric material is bonded to the core; wherein said bond coatcomprises a thermally sprayed material selected from a group ofmaterials consisting of metals, metal alloys, ceramics and cermets; andwherein said bond coat further comprises a thermally sprayed coating ofmaterial that is sufficiently permeable for diverting bond-degradingfluids away from the bond line.
 2. The roller of claim 1, wherein thewherein the of the bond coat is either a metal wire, a rod, or a powdermaterial.
 3. The roller of claim 1, wherein the wherein the bond coatcomprises a plurality of coating layers.
 4. The roller of claim 1,wherein the core is made of metal.
 5. The roller of claim 1, wherein thebond coat has a surface roughness in a range from at least approximately401 microinches R_(a) to approximately 2000 microinches R_(a).
 6. A Theroller of claim 1, wherein the bond coat further comprises a singlelayer having a thickness of at least 35 mils.
 7. The roller of claim 6,wherein the single layer is comprised of a plurality of coatings of alike material resulting from several passes of thermal spraying.
 8. Theroller of claim 1, wherein the bond coat further comprises at least twolayers, one of the two layers having a greater density and less porositythan another one of the two layers.
 9. The roller of claim 8, whereinsaid one of the two layers has a thickness in a range from about 5 milsto about 15 mils; and wherein said other one of the two layers has athickness from about 20 mils to about 125 mils.
 10. The roller of claim1, wherein the bond coat further comprises a first layer and a secondlayer which are both permeable, wherein the second layer is coarser thanthe first layer for the purpose of assisting mechanical bonding to theelastomeric layer; and wherein the second layer has greater permeabilitythan the first layer.
 11. The roller of claim 1, wherein the elastomericmaterial is either silicone rubber or urethane.
 12. The roller of claim1, wherein the core is made of a fiber-reinforced resinous compositematerial.
 13. The roller of claim 1, wherein the bond coat is a 300series or 400 series stainless steel alloy.
 14. The roller of claim 1,wherein the bond coat is a nickel-chromium alloy.
 15. The roller ofclaim 1, wherein the bond coat is a nickel-based alloy.
 16. The rollerof claim 1, wherein the elastomer is applied by casting, extrusion, orwrapping with calendered rubber sheets.
 17. The roller of claim 1,wherein the elastomeric material is based on a natural or syntheticelastomer.
 18. The roller of claim 1, wherein chemical bonding agentsare used to assist bonding between the thermal spray bond coat and theelastomeric covering material.
 19. The roller of claim 1, wherein thethermally sprayed bond layer is applied over an existing thermallysprayed bond layer.
 20. A roller comprising: a roller core; a bond coatbonded to the core to provide an outer surface roughness; a coat ofelastomeric material bonded to the bond coat to form a bond line alongwhich the elastomeric material is bonded to the core; wherein said bondcoat comprises a thermally sprayed material selected from a group ofmaterials consisting of metals, metal alloys, ceramics and cermets; andwherein said bond coat further comprises a thermally sprayed coating ofimpermeable material, wherein the bond coat has a thickness of a range 3mils to 250 mils.